1. Field
The present disclosure generally relates to the fabrication of composite structures, and deals more particularly with a method and apparatus for automated fabrication of composite fillers used to fill a cavity between assembled composite members.
2. Background
Assembled composite structures sometimes contain cavities that must be filled in order to maximize the strength and/or rigidity of the structure. For example, two or more composite members with radiused edges may be assembled together such that their adjacent radiused edges form a radiused cavity. In order to fill the cavity and strengthen the structure, a pre-formed radius filler is placed in the cavity and bonded to the members.
Fillers, including radiused fillers used in primary aircraft structures such as I-beams and spars, must have tight dimensional specifications and a high level of both geometric accuracy and compaction in order to maximize structural strength and avoid resin pooling or the development of voids during part cure. Radius fillers may be fabricated by hand, however this production technique is time consuming, labor intensive and may yield radius fillers that are inconsistent, and/or do not meet required specifications.
Automated machines have been proposed for producing radius fillers using a pultrusion process in which prepreg unidirectional tape is continuously pulled through one or more forming dies which form the tape to the desired cross-sectional radius filler profile. However, these automated radius filler machines are not currently capable of producing radius fillers to stringent specifications for peak definition, even ply transitions, symmetry and the absence of loose fibers. These deficiencies are caused, in part, by inadequate pre-conditioning and/or poor forming die design.
Further problems may be encountered during automated pultrusion of variable radius fillers which have cross-sectional profiles that vary along their length. A pultruded varying cross sectional profile requires the use of composite tape having a width that changes at transition points forming “tabs” along one edge of the tape. Although a portion of the width of the tape is held in tension during the pultrusion process, the remaining portion containing the tabs is not tensioned, and thus “droops”. The inability to tension and integrate the tabs into the filler leads to uneven ply transitions and fiber distortions created by the interaction of the leading edge of the tabs with forming die surfaces, both of which may affect radius filler quality. Additionally, the existence of tabs along the edge of the radius filler results in asymmetry which create problems when installed.
Accordingly, there is a need for a method and apparatus for producing composite radius fillers for high-performance applications that improve part-to-part uniformity and consistency. There is also a need for a method and apparatus to improve the surface finish, dimensions, cross sectional profile and integration of tabs in variable composite radius fillers.